Japanese only has 48 main sounds to play with. English has 26 letters but the combinations thereof produces many more sounds. So since there are fewer sounds in Japanese, there are many words that have the same pronunciation but different meanings. For example, shi could mean death, poem, city, a man etc.

Also, "mon" is sort of a childish way to express dissatasfaction...
学校に行きたくないもん！ I don't wanna go to school!
If you under 7 (and I assume you are), this can be used, but usually when your trying to be funny.